This invention relates to a novel CRT (cathode-ray tube) having a layer of carbon particles on a metallized viewing screen and to methods of preparation thereof.
U.S. Pat. Nos. 3,703,401 to S. B. Deal et al. and 4,025,661 to J. J. Moscony et al. each disclose a CRT comprising a screen support, a luminescent viewing screen on the support, a light-reflective metal layer on the screen and a carbon-particle layer of amorphous carbon and/or graphite on the metal layer. The carbon-particle layer may absorb heat that is radiated from an associated aperture mask, or may absorb electrons that are scattered from, or generated by, the electron beam or beams that excite the viewing screen. The carbon-particle layer does not include a permanent binder although it is usually made using a temporary organic binder, which is removed during a baking step designed to oxidize or otherwise volatilize organic matter from all of the layers on the screen support.
It has been found that the carbon-particle layer is a source of loose particles after the baking step. After the structure is assembled into an operative CRT, such loose particles can lead to problems of high-voltage stability in the CRT. Thus, it is desirable to include a permanent binder in the carbon-particle layer. The above-cited Moscony et al. patent points out why a metal-ion residue in the carbon-particle layer is undesirable. In addition, any addition to the carbon-particle layer which reduces the luminescent brightness of the screen by more than 5% is undesirable. Thus, the obvious choices of a permanent binder for the carbon-particle layer are unacceptable.